Hello Florian!
If you lost your front page boxes, open Customizing ▸ Frontpage Boxes->General
and set Visibility on Front Page. https://prnt.sc/h8n9of
For the slider, edit your front page (Dashboard->Pages->your front page/edit)
then scroll down to the bottom of the page, and enable slider on the page in “Agama options” meta box.
https://prnt.sc/h8nc0i
https://prnt.sc/h8nbap
For the Header image, open theCustomizing->Header->Image & Video
menu, and pick your pages from the drop-down box, where you want to show the header image.
https://prnt.sc/h8nc0i
Best Regards